When a coherent beam of light passes through a diffuser and is projected onto a surface, a primary speckle pattern can be observed on the surface. The primary speckle is caused by interference among different components of the diffused beam. The term “primary speckle” is used in this sense in the present patent application and in the claims, in distinction to secondary speckle, which is caused by diffuse reflection of coherent light from the rough surface of an object
Hart describes the use of a speckle pattern in a high-speed 3D imaging system, in Taiwanese Patent TW 527528 B and in U.S. patent application Ser. No. 09/616,606, whose disclosures are incorporated herein by reference. The system includes a single-lens camera subsystem with an active imaging element and CCD element, and a correlation processing subsystem. The active imaging element can be a rotating aperture which allows adjustable non-equilateral spacing between defocused images to achieve greater depth of field and higher sub-pixel displacement accuracy. A speckle pattern is projected onto an object, and images of the resulting pattern are acquired from multiple angles. The images are locally cross-correlated using an image correlation technique, and the surface is resolved by using relative camera position information to calculate the three-dimensional coordinates of each locally-correlated region.
Another speckle-based 3D imaging technique is described by Hunter et al., in U.S. Pat. No. 6,101,269, whose disclosure is incorporated herein by reference. A random speckle pattern is projected upon a 3D surface and is imaged by a plurality of cameras to obtain a plurality of two-dimensional digital images. The two-dimensional images are processed to obtain a three-dimensional characterization of the surface.